1. Field of the Invention
The invention relates generally to pavers, such as, for example, asphalt pavers used in continuous paving operations for long strips of pavement, such as on highways, airport runways, parking lots and the like. More particularly, the invention relates to grade reference systems which are instrumental in controlling the grade of the paved surfaces.
2. Description of Related Art
All pavements designed particularly for vehicles moving at high speeds typically include controlled transverse slopes to assure proper water run-off. Grade controls to assure compliance to surveyed grades along the lengths of pavement strips are equally important for pavements used for high speed operations, such as interstate highways and airport runways. Straight runs are particularly important with respect to airport runways. Highways require smooth transitions from straight runs into ascending or descending grades. A particular problem in the prior art relates to effectively mounting grade reference sensors on pavers using screed extensions to enable the pavers of laying down pavement of various widths. Accordingly, state of the art pavers have the capabilities to effectively more than double their standard paving widths by the use of screed extensions.
Typically, the use of screed extensions brings about a need to position grade reference systems outward with respect to the centerline of the paver using the screed extensions. For example, typical grade sensors make reference to and indicate deviations from one of various types of surveyed grade references, such as direct sensing of an existing grade by a sensing shoe, by referencing to a string line on a travelling ski, or to string lines which are strung off to the sides of the path of the paver to indicate the surveyed grade of the pavement. Thus, the grade reference sensor necessarily has to be adjusted outward from the centerline of the paver, as the paving width of the paver is increased by additions to the screed.
According to a current practice, the grade sensors are usually supported by the front part of the pull arms by which the screed of the paver is pulled along, such front part being near the pull point through which the pulling force of a tractor is transmitted to the screed. A grade sensor positioned near the screed receives only attenuated deviation indications of upcoming grade changes. With substantially no anticipatory grade change indications, transitions from one indicated grade to another tend to be more abrupt, as some floating screeds may be slow to react to grade changes, the actual grade change may lag the desired grade. Abrupt changes are likely to be undesirable for paving highways or runways designed for handling high speed traffic. For an anticipation of a grade change and a smoother transition from one grade to another, the grade sensors are preferably mounted somewhere intermediate the front and rear ends of the pull arms.
The forward ends of the pull arms present no particular difficulty for mounting support arms of standard length for grade sensors. Such standard support arms would be applicable when the paver is used without screed extensions and the grade reference line is located about three feet laterally away from the pull arm. When the paver is used with screed extensions, however, supporting the grade sensors becomes more difficult. The difficulty results from the lack of a support base for attaching the cantilevered length of the support needed to position the grade sensors laterally outside of the width to be paved. The forward ends of the rear pull arms are typically located laterally on both sides of the tractor unit. Up to now, long extensions for supporting the grade sensors have had to be strengthened by, for example, overhead braces which were attached at their base ends to the superstructure of the tractor unit. Frequently more than one length of extension and its respective brace may be needed to provide the proper support for grade control sensors suspended at different widths. The grade sensor supports may, consequently, present a substantial inventory of accessory hardware, all of which may need to be held in readiness at job sites. Maintaining the readiness of the additional extensions and braces adds to the cost of paving operations.
Also, it has been recognized that a grade reference sensing position just ahead of the screed offers a more positive control over transverse changes between opposite sides of the screed than a grade reference sensing position near the pull point of the screed pull arm on the tractor. A sensing position referenced substantially to the screed will give a continuous indication of the position of the screed to the grade reference, hence the string line. If the quality of a paving job is to be judged by the accuracy to which the paver lays the pavement to the grade reference, an anticipatory grade change away from the current string line reference is undesirable.
On the other hand, a sensor location on the pull arms near the screed has been found to be sensitive to lateral flexing of the pull arms during the paving operation. Such flexing about longitudinal axes of the pull arms is particularly noticeable when the screed is adjusted for a "crown", such that the center of the screed is raised with respect to its ends.
It is therefore desirable to overcome problems that relate to shifting of a grade sensor because of a flexure of a pull arm, to accommodate extensible screeds and resulting transverse shifts in the location of grade reference lines, and to provide adaptability to changes in requirements and to achieve a balance between response to anticipatory grade changes and maintaining an optimum degree of control over screed elevation with respect to a string line or similar grade reference.